Another contribution from Wikipaedia:  The term “psychedelic” is derived from the Ancient Greek words psychē (ψυχή, “mind”) and dēloun (δηλοῦν, “to make visible, to reveal”),translating to “mind-revealing”.

A psychedelic experience is characterized by the striking perception of aspects of one’s mind previously unknown, or by the creative exuberance of the mind liberated from its ostensibly ordinary fetters. Psychedelic states are an array of experiences including changes of perception such as hallucinations, synesthesia, altered states of awareness or focused consciousness, variation in thought patterns, trance or hypnotic states, mystical states, and other mind alterations. These processes can lead some people to experience changes in mental operation defining their self-identity (whether in momentary acuity or chronic development) different enough from their previous normal state that it can excite feelings of newly formed understanding such as revelation, enlightenment, confusion, and psychosis.